It is known from U. S. Pat. No. 3,644,482 that tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxymethyl]methane(h ereinafter sometimes referred to as "compound I") can be produced by transesterification between a lower alkyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate and pentaerythritol in the presence of a base catalyst, such as an alkali metal lower alkoxide.
Recent investigations into the crystal structure of compound I have revealed that compound I can occur in at least four crystal forms, namely alpha, beta, gamma and delta (U.S. Pat. No. 4,405,807). Among them, alpha crystals, when crystallization is carried out in the conventional manner, tend to become a fine powder, making stirring impossible, as described in the above-cited U.S. patent. As a result, crystals obtained have poor purity and color features. Therefore, from the production process and product handling viewpoints, among others, the beta crystal structure is a suitable one. For the production of beta crystals, a process is known from Laid-open European Patent Specification No. 244,361 which comprises dissolving the transesterification product in a water-soluble solvent, such as methanol or ethanol, which contains 2-10% by weight of water and, after clarifying and filtering as necessary, cooling the solution for gradual temperature fall. When the crystallization liquor is cooled gradually, however, the crystal structure changes in part to the alpha structure, which is unfavorable from the product handling viewpoint, as mentioned above. In addition, when the slurry obtained after crystallization is maintained under stirring, the crystal structure change from beta to alpha progresses with the lapse of time. Thus, it is very difficult to produce beta crystals on a commercial scale in a well-reproducible and stable manner by the crystallization process described in the above-cited laid-open European patent specification. When beta crystals are stored in a wet state at room temperature, the crystal structure changes to alpha. In view of this fact as well, it is very difficult to produce beta crystals stably on a commercial scale.
It is an object of the invention to provide an industrially advantageous process for obtaining beta crystals of compound I in a stable manner.